An operating cycle of such an engine, as is well known in the art, consists of four phases which in the 4-stroke Otto cycle correspond to respective piston strokes, namely an intake phase for the aspiration of an explosive air/fuel mixture, a compression and ignition phase, an expansion or power phase and an exhaust phase. Conventional rotary valves comprise a pair of shutters driven by the engine to rotate in close contact with each other about a common axis in synchronism with the reciprocation of the associated piston, these shutters being provided with respective apertures which register with each other during the exhaust phase and during the immediately following intake phase to unblock first an outlet port and then an inlet port of the corresponding cylinder. See, for example, British Pat. No. 151,994; reference may also be made to German Pat. No. 678,268 and to Austrian Pat. Nos. 184,777 and 307,158.
It is convenient to install such a rotary valve directly in the cylinder head, with its shutters centered on the cylinder axis, but this creates certain problems concerning the location and the size of the inlet and outlet ports which on the one hand should have a large enough cross-section to handle the aspirated and expelled gas masses and on the other hand should be sufficiently spaced apart to minimize thermal interaction. The valve apertures of the shutters registering with the outlet port in the exhaust phase and with the inlet port in the intake phase must, of course, also be so dimensioned as not unduly to throttle the gas flow during these two phases.